how to get the IP address and port number from addrinfo in unix c

Question

i need to send some data to a remote server via UDP in a particular port and get receive a response from it. However, it is blocking and I get not response. I needed to check if the addrinfo value that I get from the getaddrinfo(SERVER_NAME, port, &hints, &servinfo) is correct or not. how do i get the ip address and port number from this data structure. i know that inet_ntop(p->ai_family, get_in_addr((struct sockaddr *)p->ai_addr),s, sizeof s) gives me server IP address but i also need to make sure the port number is correct. (I am using the method in Beej's guide.)

Answer 1

You do something similar to what Beej's get_in_addr function does:

// get port, IPv4 or IPv6:
in_port_t get_in_port(struct sockaddr *sa)
{
    if (sa->sa_family == AF_INET) {
        return (((struct sockaddr_in*)sa)->sin_port);
    }

    return (((struct sockaddr_in6*)sa)->sin6_port);
}

Also beware of the #1 pitfall dealing with port numbers in sockaddr_in (or scokaddr_in6) structures: port numbers are always stored in network byte order.

That means, for example, that if you print out the result of the "get_in_port" call above, you need to throw in a "ntohs()":

printf("port is %d\n",ntohs(get_in_port((struct sockaddr *)p->ai_addr)));